1. Field of the Invention
This invention relates to a method for recording an electron microscope image and reading out the recorded electron microscope image. This invention particularly relates to an electron microscope image recording and read-out method wherein an electron microscope image is recorded with a high sensitivity, and the recorded electron microscope image is read out as an electric signal which can then be subjected to various types of image processing.
2. Description of the Prior Art
There have heretofore been known electron microscopes wherein an electron beam which has passed through a sample is refracted with an electric field or a magnetic field in order to obtain an enlarged image of the sample. A diffraction pattern of a sample is formed on a back focal plane of an objective lens in the electron microscope as a result of the passage of the electron beam through the sample, and an enlarged image of the sample is formed by interference of the diffracted waves. Therefore, the enlarged image (scattering image) of the sample may be projected with a projection lens so that it can be observed. Also, if the diffraction pattern on the back focal plane is projected, an enlarged diffraction pattern of the sample may be observed. If an intermediate lens is disposed between the objective lens and the projection lens and the focal length of the intermediate lens is adjusted, the aforesaid enlarged image (scattering image) or the diffraction pattern may be observed as desired.
In general, in order for the enlarged image or the diffraction pattern (both the enlarged image and the diffraction pattern will hereinafter be generically referred to as the electron beam image) to be viewed, photographic film has heretofore been disposed at the plane, on which the projection lens forms the image, in order to expose the photographic film to the electron beam image. Alternatively, an image intensifier has heretofore been used in order to intensify and project the electron beam image. As another alternative, a fluorescent screen has heretofore been used in order to convert the electron beam image into a visible image. The visible image is recorded with a television camera. However, the method wherein photographic film is used has a drawback in that the sensitivity of the photographic film with respect to the electron beam is low and the development process is therefore difficult. The method wherein the image intensifier or the television camera is used is also disadvantageous in that image sharpness is low and the image is readily distorted.
Also, the electron beam image is often subjected to image processing such as gradation processing; frequency response enhancement processing; density processing; subtraction processing or addition processing; reconstruction of a three-dimensional image by Fourier analysis; image analysis for image conversion into a binary image or for grain size measurement; or diffraction pattern processing for analyzing crystals or for measurement of the grating constant, transition and grating defects. In such cases, a microscope image recorded on photographic film has heretofore been detected with a microphotometer and converted into an electric signal, which is then subjected to A/D conversion or the like and processed with a computer. Performing such operations is very troublesome.
In view of the above circumstances, the applicant proposed in U.S. Pat. No. 4,651,220 and U.S. patent application Ser. No. 251,982 a novel method for recording and reproducing an electron microscope image wherein the electron microscope image is recorded and reproduced with a high sensitivity and with good image quality, and wherein an electric signal representing the electron microscope image is directly obtained so that various kinds of processing are facilitated. Basically, the proposed method for recording and reproducing an electron microscope image comprises the steps of (i) exposing a two-dimensional sensor, such as a stimulable phosphor sheet, which is capable of storing energy from an electron beam thereon, to an electron beam which has passed through a sample in a vacuum in order to store the energy from the electron beam on the two-dimensional sensor, (ii) thereafter exposing the two-dimensional sensor to light or heat in order to release the stored energy in the form of emitted light, (iii) photoelectrically detecting the emitted light in order to obtain an image signal, and (iv) using the image signal in order to reproduce an electron beam image of the sample.
The aforesaid two-dimensional sensor is constituted of a material which is capable of temporarily storing at least part of the energy from an electron beam thereon when exposed to the electron beam, and then releasing at least part of the stored energy in a detectable form such as light, electricity or sound upon excitation by an exterior source. As the two-dimensional sensor, stimulable phosphor sheets as disclosed in, for example, U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395 are particularly suitable. Specifically, when certain kinds of phosphors are exposed to radiation such as an electron beam, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to radiation is exposed to stimulating rays such as visible light, the phosphor emits light in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor. The term "stimulable phosphor sheet" as used herein means a sheet-like recording material comprising the aforesaid stimulable phosphor. In general, a stimulable phosphor sheet is composed of a supporting material and a stimulable phosphor layer overlaid on the supporting material. The stimulable phosphor layer comprises an appropriate binder and the stimulable phosphor dispersed therein. In cases where the stimulable phosphor layer is self-supporting, the stimulable phosphor layer can form the stimulable phosphor sheet by itself. Examples of stimulable phosphors for constituting the stimulable phosphor sheet are described in detail in U.S. Pat. No. 4,651,220.
As the two-dimensional sensor, it is also possible to use a thermal phosphor sheet as disclosed in, for example, Japanese Patent Publication Nos. 55(1980)-47719 and 55(1980)-47720. A thermal phosphor sheet is composed of a sheet-like recording material comprising a phosphor (thermal phosphor) which is caused mainly by the effect of heat to release energy, which was stored thereon during its exposure to radiation, as thermal fluorescence.
With the aforesaid method for recording and reproducing an electron microscope image wherein an electron microscope image is stored on a two-dimensional sensor such as a stimulable phosphor sheet, the electron microscope image can be recorded with a high sensitivity. Therefore, the energy of the electron beam used to record the electron microscope image can be decreased, and damage to the sample can be minimized. Also, with this method for recording and reproducing an electron microscope image, it becomes very easy to carry out image processing, such as gradation processing and frequency response enhancement processing, on the electron microscope image. Also, when the electric signal is fed into a computer, it becomes possible quickly and simply to carry out diffraction pattern processing, reconstruction of a three-dimensional image, and image analysis for image conversion into a binary image or the like.
In the aforesaid method for recording and reproducing an electron microscope image, it is desirable for a visible electron microscope image to be reproduced which has good image quality and which can serve as an effective tool in, particularly, the efficient and accurate analysis of a sample. For this purpose, in the course of reading out an electron microscope image from a two-dimensional sensor (i.e. in the course of detecting light emitted by the two-dimensional sensor), read-out conditions such as a read-out gain and a scale factor (latitude) should be set to values suitable for the image input information stored on the two-dimensional sensor. In order to satisfy this requirement, the method disclosed in, for example, Japanese Unexamined Patent Publication No. 62(1987)-222556, will be considered, in which method a preliminary read-out operation (hereinafter referred to as the "preliminary readout") is carried out. Specifically, before a final read-out operation (hereinafter referred to as the "final readout") is carried out in order to obtain an image signal which is used to reproduce a visible electron microscope image, a preliminary readout is conducted in order approximately to ascertain the electron microscope image stored on the two-dimensional sensor. On the basis of the approximately ascertained information about the electron microscope image, the read-out conditions for the final readout and/or image processing conditions for the processing of the image signal obtained from the final readout are set. In this manner, a visible electron microscope image having the best possible image quality can be reproduced.
However, in cases where the preliminary readout is carried out, it takes a long time to carry out the whole read-out operation. Also, because a mechanism for the preliminary readout must be provided, the read-out apparatus becomes complicated. Additionally, in cases where a plurality of two-dimensional sensors are subjected to the preliminary readout and the final readout, unless the preliminary readout carried out for a certain two-dimensional sensor is always associated with the final readout carried out for the same two-dimensional sensor, a problem will occur in that read-out conditions, which have been determined on the basis of information obtained from the preliminary readout carried out for a certain two-dimensional sensor, may be set for a final readout which is carried out on a different two-dimensional sensor. Therefore, in cases where a preliminary readout is carried out, two-dimensional sensors must be managed very carefully in the course of reading out electron microscope images. Performing such management functions is troublesome.